1. Field of the Invention
The present invention relates to an electrophotographic process utilizing electrostatic separation and an apparatus therefor.
2. Description of the Prior Art
There are already known various electrophotographic apparatus for forming an image corresponding to an original image, utilizing an image bearing member such as a photosensitive drum or an insulating drum. For example, in electrostatic copiers, a visible image corresponding to an original image if formed by process on a drum- or belt-shaped photosensitive member, and is then transferred onto a transfer sheet to obtain a final copy. For such image transfer there is known the so-called electrostatic separating method, in which corona discharge of a polarity opposite to that of the charge retained by the image-constituting coloring particles (hereinafter called toner) is applied to the rear face of the transfer sheet, thereby depositing the toner on the transfer sheet by the electrostatic attractive force, and the charge on the rear face of the transfer sheet is subsequently dissipated for example by corona discharge to separate the transfer sheet from the photosensitive member by means of the rigidity or weight of the transfer sheet itself.
Conventionally, such electrostatic separating process has only been applicable to electrophotographic processes utilizing a two-layered photosensitive member composed of a photoconductive layer and a conductive substrate, such as disclosed in the U.S. Pat. Nos. 3,357,400, 3,575,502, 3,870,515 and 3,998,536. Such method has not been practically applicable to a three-layered photosensitive member composed of a surfacial insulating layer, a photoconductive layer and a conductive substrate, since the surfacial insulating layer constitutes a complete barrier against the transfer of charge and involves therefore a narrower latitude for the balance of the applied and removed charge for sheet separation than in the two-layered photosensitive member which experiences charge decay in the dark or in the light and the charge transfer effect by the majority charge carrier.
However, the electrostatic separating method combined with the conventional two-layered photosensitive member still has certain drawbacks. One of such drawbacks is the lack of stability in the sheet separation. In fact the sheet separation may become difficult because of a fluctuation in the balance of the amount of charge applied at the image transfer and that of removed charge at the sheet separation, caused, for example, by the presence of light or dark potential on the photosensitive member, presence of a high or low moisture in the atmosphere, or the use of a heavy or light transfer sheet.
Another drawback of the electrostatic separating process utilizing the two-layered photosensitive member is related to the image quality of the final copy obtained on the transfer sheet. In the case that the toner constituting the visible image has a low volume resistivity for example in a range of 10.sup.8 -10.sup.10 .OMEGA..cm or in case the toner has a small charge for example in a range of 2-5 .mu.C/g, the toner once attracted to the transfer sheet at the image transfer step is returned to the photosensitive member under the effect of an inverse electric field at the sheet separating step, thus only providing a disturbed image with an insufficient density on the final copy. Such phenomenon is generally known as an incomplete image transfer. Such incomplete image transfer tends to appear particularly at the leading end of the image where the sheet separation is initiated. This fact is therefore well known as the leading end image loss in the electrostatic sheet separating method.
FIGS. 1A and 1B show the position of the transfer sheet with respect to the photosensitive member respectively at the start of and in the course of sheet separating step, wherein shown are a two-layered photosensitive member 1, a visible image 2, a transfer charger 3, and a separating charger 4. At the start of the sheet separating step, a leading end portion of a transfer sheet P, having no preceding paper, is subjected to charge elimination for sheet separation while it is adhered to the drum 1 and is therefore separated therefrom at a point .alpha. which is relatively deep in the charge eliminating area. Such sheet separation gives rise to an incomplete image transfer. In a succeeding portion b, as shown in FIG. 1B, the sheet P is separated from the drum 1 at a point .beta. close to the entrance of the charge eliminating area because of the weight of the already separated sheet portion. Consequently the transfer sheet P is separated from the drum 1 during the charge elimination, maintaining a satisfactory image on the final copy. Such image loss at the leading end is encountered particularly in a transfer sheet of low rigidity or of low weight.
The incomplete image transfer and the image loss at the leading end explained above tend to appear when the toner constituting the visible image retains a small amount of charge. Since the Coulomb force generated by the charge retained by the toner and by the charge applied to the transfer sheet at the image transfer is responsible for forming a regular arrangement of the toner on the transfer sheet, a smaller charge on the toner not only reduces the efficiency of image transfer but also reduces the force anchoring the transferred toner on the transfer sheet, thus promoting the disturbance in the image and the image loss at the leading end in the electrostatic sheet separating method because of the presence of an inverse electric field repelling the toner from the transfer sheet at the charge eliminating step. A series of experiments conducted by the inventors has proved that the two-layered photosensitive members, particularly the widely used ones based on selenium or zinc oxide, are essentially unfavorable with respect to the amount of charge retained by the toner and to the application of an electric field at the charge eliminating step in the electrostatic sheet separating method because of the following reasons:
(1) In a two-layered photosensitive member, the charge retained by the toner is of the polarity opposite to that of the majority carriers in the photosensitive layer, so that the photosensitive member functions as a conductor to the toner, thus inevitably attenuating the charge on the toner. Such phenomenon becomes particularly apparent in relatively conductive toner, having a specific resistivity in a range of 10.sup.8 -10.sup.10 .OMEGA..cm.
(2) In the electrostatic separating method, the charge applied at the image transfer is of the same polarity as that of the majority carrier in the photosensitive layer but the charge applied at the sheet separation is of the opposite polarity. This fact suggests that the photosensitive member functions as an insulator at the image transfer step but as a conductor at the sheet separating step. Consequently the transfer sheet is placed in a relatively strong electric field by the conductive nature of the photosensitive member, particularly at the leading end portion of the image, so that the once transferred toner tends to return to the photosensitive member.
The above-mentioned phenomenon (1) is related to the attenuating time of the charge of the toner and becomes apparent in a relatively slow process, while the phenomenon (2) becomes apparent in a high-speed process in which the image transfer and the sheet separation are both conducted under strong electric fields.